The present invention relates to a method for creating three dimensional bodies, relief images or patterns and colored bodies, or a combination of these, using photopolymers or photocolorizable polymers. In particular, the invention describes a method for generating three-dimensional and colored objects using a digital light processing technique to selectively image a volume or "voxel" of photopolymer (liquid, gel or solid) and/or photocolorizable composition to form a solid polymer with differential control of the properties (depth, color density, mechanical strength, hardness, cross-linking, functional group conversion, tackiness, etc.) such that the selected property varies across the xy array. Any property of the material that is sensitive to the degree of cure of the polymer can be differentiated across the xy array. Other properties such as the selective formation of color may also be differentiated using this technique. In addition, this invention describes a method of forming polymeric images or patterns without the use of a separate mask.
The use of photopolymerizable resins to create patterns is known. One process for generating polymeric patterns, requires making a mask either through photographic techniques on film or ionographic techniques on glass. The mask is placed between a radiation source such as a UV or visible lamp and the photopolymeric material (liquid, gel or solid) such that the desired light pattern passes through the mask to generate a solid polymer in the desired pattern. These mask techniques are used in photoresists as well as for making 3D objects.
Additionally, methods for creating solid three-dimensional objects based on stereolithography and additive fabrication processes are also known. The stereolithography process is based on the technique of successively building up cross-sectional "layers" of a photocurable resin that have been selectively cured using lasers or other actinic radiation, as described fully in U.S. Pat. Nos. 4,929,402 and 5,236,637 to Hull. Briefly, a prior art stereolithographic apparatus is comprised of a vat of liquid polymer, a platform positioned in the vat that may be precisely lowered in a stepwise manner, and a device for selectively directing actinic radiation to the surface of the polymer. The platform is positioned near the top of the vat so that a thin layer of a polymerizable composition coats the surface of the platform. Actinic radiation, generally in the form of laser light, is selectively directed to the top of the platform in a pattern corresponding to a cross-section of the object being created. Accordingly, the composition is polymerized in the desired pattern thereby creating a single cross-sectional "slice" of the object. The platform is then lowered a precisely measured distance and the process is repeated, continually building up cross-sectional layers of the object, until the desired object is fabricated.
Several innovations have been made over the last few years which improve upon this basic method in various ways. While most of the fabrication methods are based on laser irradiation techniques, there are also fabricators available from Cubital (Israel) which utilizes a mask and broad spectrum UV lamp. Additionally, in our U.S. Pat. Nos. 5,514,519, 5,677,107 and U.S. application Ser. No. 08/603,642 filed Feb. 20, 1996 we describe methods for the production of colored three-dimensional objects.
Recent innovations in digital light processing (DLP) technology have provided new tools for use in industry. For example, in WO 98/06560, entitled Apparatus for Automated Fabrication of Three-Dimensional Objects, and Associated Methods of Use owned by SRI International, a stereolithography method is disclosed for making three-dimensional objects using a binary light switch or digital micromirror device (DMD) to selectively photoexpose layers of photopolymerizable compositions. The method disclosed in this application has some advantages over prior art methods in that a laser is not used to provide the actinic radiation to cure the polymer. However, the SRI application is limited to the fabrication of multi-layer objects. It does not disclose a method for varying grayscale and thereby controlling exposure of the photopolymerizable compositions and thus controlling material properties as a function of the xy array, nor does it describe a method for forming bodies having color differentiation.
While stereolithography methods, particularly those based on laser techniques, are effective for making three dimensional objects from very thin layers (typically 25-150 micron layers) they are not effective for thicker layers. Lasers are also expensive, have relatively short lifetimes and high power requirements. Additionally, lasers are somewhat restrictive in that they offer curing only at a single wavelength. A broad spectrum light source could also be used as an irradiation source, and while a filter or monochromator can be used to select a specific wavelength, it is practically impossible to select several different wavelengths or intensities in a defined pattern.
There are several advantages to utilizing a DMD system over other irradiation methods. Higher brightness, higher contrast and higher resolution can be obtained than is possible with a regular projector system. Because the DMD mirrors are reflective, they offer higher light irradiance than a typical LCD based system. Additionally, the DMD based illuminators can project wavelengths closer to the UV (400 nm) than is possible with LCD technology. They are also less expensive to maintain than a laser. In addition, none of the prior art methods disclose a process which allows for substantial control of the materials properties or colorization.
Another characteristic of the DMD technology is that it eliminates the need for a contact mask. While contact masks are used for many irradiation applications, there are several drawbacks to using masks. For one thing masks are expensive and time consuming to make, particularly for detailed patterns in which masks must be inspected and repaired before use. Speed and ease of use are also sacrificed when several masks are required. In addition, some polymer properties vary when photopolymerized in the presence of a contact mask, for example light scattering often occurs with masks leading to polymeric patterns of lower resolution, less sharpness and different surface properties.
Digital masks or images projected from a DMD illuminator have the advantage that they may be easily modified. Another advantage is that they can be used in processes which would be impractical or impossible for use with a contact mask. For example, when a contact mask is used the resin or surface is covered, this prevents contact with another gas, liquid or solid which is important in the subsequent chemical step. Use of a digital mask may speed up the process by allowing what would have been two individual steps to occur simultaneously. A DMD illuminator enables one to vary grayscale, i.e., the intensity or total exposure to radiation at each spot on the exposed surface and thereby control properties in the xy Plane or in the z direction.
Accordingly, it would be desirable to provide a method for creating three-dimensional objects, polymeric relief images and patterns quickly, easily and inexpensively. Additionally, it would be desirable to have a method for creating multicolored three-dimensional objects or patterns having color variations throughout the object.